1. Field of the Invention
The present invention relates to reproduction of information from a storage medium, in which digital video information and other information can be recorded at high density. More specifically, the present invention relates to a technique for reproducing information from a high-density optical disc medium with information contained in track wobbles.
2. Description of Related Art
In recent years, optical disc media continue to increase in density. In general, in a recordable optical disc medium, track grooves are preformed. Information is recorded along each of the track grooves, that is, on the track groove or an area in-between track grooves (so-called “land”). Each of the track grooves is formed as a wave-like wobble such as a sine wave. Information is recorded in synchronization with clocks generated according to the wobble period. In addition, in order to record information in a specified position of the optical disc recording surface, addresses are provided along the track grooves. Configuration of addresses will be described as follows with examples given.
First of all, the first example, Japanese Laid-open Patent Publication No. 6-309672, is a technique to form local and intermittent track grooves with wobbles pre-formed and make them reproducible as so-called pre-pits. This technique adopts a configuration in which address-dedicated area and data-dedicated area (for recording information) coexist. The second example, Japanese Laid-open Patent Publication No. 5-189934 is a technique for frequency-modulating wobbles and describing address information, that is, sub-information. According to this technique, data information is overwritten onto address information. Furthermore, the third example, Japanese Laid-open Patent Publication No. 9-326138, is a technique to form pre-pits between track grooves adjacent to each other to form addresses.
However, when taking still more increased density that would occur in the future into account, all of the above techniques have problems, respectively. First of all, in the configuration according to the first example, the data area is reduced by the address area to be secured (so-called, overhead) and thus, recording capacity must be reduced.
Next, with respect to the second example, first of all, groove wobble primarily aims at generating clocks of recorded information and it is desirable that groove wobble is formed by a single frequency. Because a high-accuracy recording clock signal can be generated by simply synchronously multiplying the wobble reproduction signal with a single frequency, by using phase locked loop (PLL), etc. However, in the case groove wobble contains a plurality of frequency components, PLL tracking band must be lowered as compared to the case of the wobble with a single frequency in order to avoid pseudo-lock of the phase locked loop. Accordingly, phase locked loop cannot satisfactorily follow disc motor jitter or jitter generated by disc eccentricity, and as a result, jitter may remain in the recorded signal. In addition, if a recorded film formed on an optical disc recording surface is, for example, a phase-changed film, an S/N ratio of the recorded film may be lowered while rewriting is repeated. Even if the S/N ratio is lowered, noise components can be eliminated using a bandpass filter of narrow band when the groove wobble is generated with a single wobble frequency.
However, if groove wobble is frequency-demodulated and contains a plurality of frequencies, the filter band must be expanded for the frequencies. This would cause noise components to be mixed and further aggravates the jitter. From the perspective that jitter margin decreases as the recording density increases, this kind of jitter increase is not desirable.
Next, with the third example, because pre-pits naturally exert influence on adjacent tracks, it is difficult to make the pre-pit length sufficiently long or the quantity sufficiently large. In particular, for increased density, there is a fear of increasing detection errors.
In view of the above problems, the present applicant proposes an optical disc medium that gives information with a wobble form with sharp displacement toward inner periphery assigned to “1” and a wobble form with sharp displacement toward outer periphery assigned to “0” to mark an address. As one means for detecting the address information of the above-mentioned optical disc medium, there is a method for generating a carrier of wobble frequency, for example, the second harmonic, multiplying and integrating it to reproduction signals, and judging “1” or “0” by the sign. This is a technique to carry out heterodyne detection on the second harmonic contained in the reproduction signal by using the fact that waveforms with different gradients in rising or falling are attributed to the difference of phase polarity of even harmonics. The second harmonic for multiplication can be easily generated in the PLL by 2N-multiplying a frequency of the wobbles to first generate multiplied clocks, and then by N-demultiplying the multiplied clocks, for example (N is a rational number).
However, in the event that a reference wave of the second harmonic is generated by the clocks with wobble signals multiplied as described above, the detection sensitivity by heterodyne detection may lower. This is because phase of wobbles shifts due to interference of adjacent track wobbles to cause phase deviation with respect to the second harmonic in the signals to be detected.